1. Field of the Invention
The present invention relates to, for example, an image measuring apparatus and an image measuring method using a single-chip color camera.
2. Description of the Related Art
The image measuring apparatus is a precise measuring instrument which images a work, detects the edge position of the work on the image, and measures the shape and dimension and the like of the work based on the edge position. A CCD (Charge Coupled Device) color camera including the CCD as the image sensor is commonly used for imaging (see, for example, Japanese application patent laid-open publication No. Hei 10-145612 (paragraph 0057, FIG. 4)).
Because the CCD senses only light intensity, the CCD alone can provide only monochrome images. A color filter over each pixel of the CCD can provide color images. For example, for color expression with R (red), G (green), and B (blue), filters of these three colors are necessary.
Information (sensitivity) of all RGB colors is necessary for one pixel to provide color image. Because only one type of color filter can reside over one pixel (that is, only one color can be allocated to one pixel), one pixel can provide only one type of color information. To provide color images in such a limitation, a three-chip (i.e. three-CCD type or three-CMOS sensor type) color camera and a single-chip (i.e. one-CCD type or one CMOS-sensor type) color camera have been developed.
The three-chip color camera has three CCDs with the same resolution. An RGB spectral prism divides incoming light into R, G, and B components. An R-band CCD senses the R component, a G-band CCD the G component, and a B-band CCD the B component. The single-chip color camera has one CCD with each pixel having thereover either one of the R, G, and B color filters. A postprocess provides pseudo-colorization by calculating the information of the absent color of R, G, and B for each pixel using the color information obtained from the adjacent pixel.
At edges of the work, however, the brightness extremely differs between adjacent pixels. In the single-chip color camera which uses the color information from the adjacent pixels to express color, therefore, the abnormal color-density change at the edge may blur the edge image or may cause a zigzag pattern. An edge position detection based on such an image will increase the measurement error, prohibiting high accuracy measurements. The current image measuring apparatus thus uses the three-chip color camera which provides high-precision images.
The three-chip color camera, however, suffers from the following drawbacks. (1) The three-chip color camera is more expensive than the single-chip color camera because it needs three CCDs, which contributes to the cost of the image measuring apparatus. (2) The image measuring apparatus includes the CCD, color camera in a movable unit. For higher positioning accuracy of the unit, the unit needs to be driven with as little force as possible. Because the three-chip color camera includes three CCDs, a CCD with the same light-receiving size as the CCD in the B/W (black and white) camera will increase the camera size. This, in turn, provides a larger unit which needs to be driven by more force. (3) During the assembly of the RGB spectral prism which is one of the manufacturing processes of the CCD color camera, dust can easily be trapped on the mating faces of the prism. The image measuring apparatus has a microscope optical system which uses a narrow incoming beam. If, therefore, the RGB spectral prism with the dust trapped splits the incoming light beam, the dust may be imaged together with the work. It is thus unpractical for the image measuring apparatus to use the three-chip color camera with the dust trapped on the mating faces of the prism. The three-chip color camera thus has a lower yield than the single-chip color camera, which also contributes to the cost of the image measuring apparatus.
It is an object of the present invention to provide an image measuring apparatus and an image measuring method which allows high accuracy measurements using the single-chip color camera as the imaging portion.